Linked from
The 15 pages that link to Orbital perturbation, each with the reason it gives.
Celestial mechanicsBroader topic: It accounts for departures from simple Keplerian paths.
Orbital elementsRelated: Perturbations make real orbital elements change over time.
Specific orbital energyRelated: Perturbations can make the two-body specific energy vary over time.
Molniya orbitRelated: Earth’s gravity field changes the orientation of an uncorrected Molniya orbit.